The ever increasing demands for high capacity communications systems has seen the wide spread employment of optical fibre networks across the world. A fundamental component for such systems is a means of converting optical pulses comprising a digital bit stream into electrical signals. This component of such a system is commonly known as an optical receiver.
The operational requirements of such a receiver are very demanding. The receiver is required to exhibit a very low noise characteristic, such that it is capable of detecting very low levels of optical input in systems employing maximum optical fibre lengths, thus requiring high gain amplification for maximum sensitivity, but is conversely required to cope with high levels of optical input in systems employing short fibre lengths, thus requiring low gain amplification. As such, the optical receiver is required to have a wide dynamic range which can only be practically achieved with some form of automatic gain control (AGC). A typical integrated circuit (IC) optical receiver 10 is illustrated in block schematic form in FIG. 1. This comprises an IC (denoted by broken line 12) including a transimpedance amplifier stage (denoted by broken line 14) with an integrator in a control loop providing AGC.
As illustrated by FIG. 1, optical input power OP.sub.IN is converted into an electrical current I.sub.IN by a PIN diode photodetector 16. This current I.sub.IN is applied as an input to the IC optical receiver 10. The input current I.sub.IN is amplified by a transimpedance amplifier (Tz Amp) 18 which converts the input current I.sub.IN into an amplified voltage output signal V.sub.OUT. To meet the requirement of wide dynamic range the output voltage V.sub.OUT of the Tz Amp 18 which is in the form of a broadband data signal and may be considered as an ac, multi-frequency signal, is rectified or peak detected by a rectifier/peak detector 20 to provide a dc signal level V.sub.REC for comparison with a pre-determined dc reference voltage V.sub.REF. The difference between the rectified/peak detected output voltage V.sub.REC and the reference voltage V.sub.REF is considered as an error signal which is amplified and integrated by a Miller Integrator 22 to provide a control signal V.sub.CONTROL. A Miller Integrator is a well known form of integrator incorporating an active device such as a transistor amplifier. The Miller Integrator 22 is required to have a high gain, in order to ensure that the error signal approaches zero (ie in order to ensure that the difference between the rectified/peak detected output voltage V.sub.REC and the reference voltage V.sub.REF becomes zero) by means of controlling the gain of the Tz Amp by varying the impedance of a feedback resistor 24.
If the rectified/peak detected output voltage V.sub.REC is smaller than the dc reference voltage V.sub.REF, then the Tz Amp 18 must operate at high gain to provide high sensitivity of the optical receiver. When the rectified/peak detected output voltage V.sub.REC becomes just greater than the dc reference voltage V.sub.REF, then the on-set of AGC occurs and continues whilst the input channel I.sub.IN increases. When the feedback resistor 24 is at a minimum the Tz Amp is operating at very low gain and approaches an overload condition.
The most effective method of controlling the gain of the Tz Amp 18 is to vary the value Rf of the feedback resistor 24, as shown by FIG. 1. This can be achieved when using such technologies as CMOS, BiCMOS or GaAs MESFET were the availability of Field Effect Transistors (FETs) allows them to be configured as variable resistors connected in parallel with the feedback resistor 24 to vary the Tz Amp gain. To achieve the highest performance receiver GaAs MESFET technology is often used but at greatest expense. Alternatively, CMOS technology can offer the lowest cost but offers the poorest performance in terms of sensitivity. A good compromise is often provided by using BiCMOS technology, where a high performance is achieved through use of bipolar transistors for low noise while the gain control is achieved by the FET connected in parallel with the feedback resistor. The use of bipolar technology can achieve sensitivities comparable to GaAs MESFET technology, but poor gain control techniques may result in either lowering of the sensitivity of the receiver or providing an inadequate dynamic range.